Two-stage self-sustaining catalytic oxidation unit



Feb. 18, 1969 Y HURKQ 7 3,428,434

TWO-STAGE SELF-SUSTAINING CATALYTIC OXIDATION UNIT Filed Sept. 15, 1964 A FIRST STAGE CERAMlC Booo n L 2 (even AIR. TEMPERATURE 800 w fl e ovEN sx /ws'r P 600 (AFTER SEcorJD s-v 8% 5260240 STAGE cERAMlc 4-00 S u! INVENTOR.

B HDAM HURKO TsME nu MHOUTES 2o 40 6 BIG y a 1+ ATTORNEY" United States Patent ()ffice 3,428,434 Patented Feb. 18, 1969 3,428,434 TWO-STAGE SELF-SUSTAINING CATALYTIC OXIDATION UNIT Bohdan Hurko, Louisville, Ky., assignor to General Electric Company, a corporation of New York Filed Sept. 15, 1964, Ser. No. 396,549 US. Cl. 23288 Int. Cl. B01j 9/04; F24c 15/00 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the self-sustaining catalytic oxidation unit disclosed and claimed in the co-pending patent application of Bohdan Hurko and Raymond L. Dills, Ser. No. 396,551, filed in the US. Patent Ofiice concurrently herewith and is assigned to the General Electric Company, the 'assignee of the present invention. In the oxidation unit of this co-pending application there is a tubular housing supporting a perforated ceramic block of cellular construction or corrugated ceramic that is coated with a catalytic material so that hot gases passing through the unit will undergo an exothermic reaction at a minimum temperature in the range between 300 F. and 400 F. until the smoke and other undesirable properties in the gases have been dispelled before they are returned to the atmosphere. Such a design works well at normal and heavy loads of grease which might be found in the baking oven of a typical home.

Normal cooking temperatures range from about 150 F. to 550 F., but a self-cleaning oven of the type disclosed in the Bohdan Hurko patent 3,121,158 is operated at heat cleaning temperatures somewhere between about 750 F. and 950 F. A regulated amount of fresh air is supplied to the oven for a complete combustion in the oxidation unit depending upon the amount of grease load. The greater the load, the greater the amount of fresh air is needed for combustion of the volatile products. If the grease load is increased beyond the capacity of the oxidation unit there may be an accumulation of an explosive mixture in the oven which will eventually become ignited when the oven temperature reaches about 700 F. Also ignition can take place within the oxidation unit causing the unit to become overheated and possibly destroying it. One way to prevent such a possibility of explosion would be to carefully meter the fresh air supply to the oven to obtain under very heavy smoking conditions a rich mixture which will not ignite. However, this oxygen deficiency allows only partial combustion in the oxidation unit and some of the smoke and other undesirable properties in the hot gases will pass through the oxidation unit.

The principal object of the present invention is to provide a two-stage catalytic oxidation unit for use with very heavy soil loads that provides partial combustion in the first stage and complete combustion in the second stage so as to prevent the possibility of ignition.

A further object of the present invention is to provide a two-stage catalytic oxidation unit with a reduced amount of primary air for the first stage and a supply of secondary air introduced after the first stage for the second stage so that under heavy smoking conditions only partial combustion can be accomplished in the first stage while complete combustion occurs in the second stage.

A still further object of the present invention is to provide a two-stage catalytic oxidation unit for reducing the temperature of the oven exhaust.

The present invention, in accordance with one form thereof, is embodied in a two-stage self-sustaining catalytic oxidation unit comprising a first and a second housing, where the second housing is supported from the first in tandem relation. Eachhousing contains a catalytic coated perforated ceramic block of cellular construction for receiving hot gases therethrough so that at a minimum temperature between 300 F. and 400 F. and above the smoke and other undesirable properties in the gases undergo an exothermic reaction and render the temperature of the ceramic block self-sustaining. A limited amount of primary air is supplied to the first housing and a limited amount of secondary air is supplied to the second housing downstream of the first housing so that only partial combustion takes place in the first housing during very heavy soil loading conditions while complete combustion is effected in the second housing. For normal loads, complete combustion may take place in the first stage.

My invention will be better understood from the following description taken in conjunction with the accompanying drawing and its scope will be pointed out in the appended claims..

FIGURE 1 is a cross-sectional elevational view of a two-stage catalytic oxidation unit comprising the present invention as it would be installed in the vent opening of a domestic baking oven;

FIGURE 2 is a cross-sectional plan view of the first stage of the oxidation unit taken on the line 2--2 of FIGURE 1;

FIGURE 3 is a time-temperature graph plotting different curves for a specific embodiment of the invention, illustrating the relationship between the oven air temperature, the temperature of the ceramic of the first stage, the temperature of the ceramic of the second stage, and the temperature of the exhaust gases leaving the oxidation unit.

Turning to a consideration of the drawings and in particular to FIGURE 1 there is shown a catalytic oxidation unit 10 embodying the present invention. It is illustrated as being vertically arranged over a vent opening 11 in a top wall 12 of an oven liner of a domestic baking oven for treating the oven exhaust gases. As is conventional in this art, the oven liner is surrounded by thermal insulation 13 such as fiberglass or the like for retaining the heat generated within the oven. The oxidation unit 10 includes a first housing 15 and a second housing 16 supported therefrom in a tandem relationship with a secondary air inlet opening 33 provided therebetween. The first housing supports a perforated ceramic block 17 of cellular construction, and the second housing 16 supports a similar block 18.

Two examples of a type of ceramic material that has been found acceptable are cordierite and alumina available from the Minnesota Mining and Manufacturing Company. It is described as corrugated ceramic as is best understood in FIGURE 2 in that it is made up of cellular construction having a configuration of multiple layers of corrugated paper arranged in a vertical stack. There are parallel spaced partitions 20 Where each adjacent pair of partitions is separated by a corrugated spaces 21 thereby establishing a perforated block of elongated cellular construction. The corrugations measure about seven corrugations per inch, the width of each cell is about .100 inch and the depth of the block is about one or two inches. This cellular construction might be described by some as depicting a honeycomb, except that the cells are not hexagonal in configuration as is the case in a true honeycomb.

A catalytic coating material such as a platinum black is applied to each ceramic block 17 and 18 and particular- 1y to the surfaces of the cells which are in contact with the exhaust gas flow. The raising of the temperature within the oven above normal cooking temperatures causes the degradation of the food soils that have become lodged on the walls thereof thereby producing corresponding gaseous degradation products which are combined with a controlled amount of ambient air that is brought into the oven cavity usually around the oven door. Such degradation products may include smoke, methane, ethane, water vapor, carbon dioxide, some free carbon, fumes and other undesirable elements. The heating of cooking greases develops smoke at temperatures of around 380 F., and it is important to be able to eliminate it and the other undesirable gaseous products before the exhaust gases are returned to the kitchen atmosphere.

Going back to FIGURE 1, the housing of the first stage of the oxidation unit includes a circular flange on the bottom edge thereof which contains several screw openings 26 for receiving fastening screws 27 which are each threaded into a mating opening in the top wall 12. of the oven liner. The housing 15 of the first stage has an inlet opening overlying the vent opening 11 of the oven liner as well as a restricted exhaust opening 29 at its opposite end that is formed by a gradually converging truncated cone formation 30. The housing 16 of the second stage also has an inlet opening 31 and an exhaust opening 32. The inlet opening 31 is formed by a gradually tapered diverging horn shaped configuration 32 which is telescopically arranged in a spaced relation down over the truncated cone of the exhaust opening of the first stage, while providing a secondary air inlet opening 33 of annular shape for the second stage to obtain complete combustion in the second stage in the event of overloaded soil conditions. The second housing 16 is supported in a tandem relation above the first housing 15 by means of radially spaced vertical straps 35 which are integral with the second housing 16 and extend down therefrom to be fastened to the outer surface of the first housing 15 as by means of fastening screws 36. The exhaust opening 32 of the second housing 16 is provided at the top of an elongated section 38 to give a chimney efiect to the oxidation unit and create a natural draft from the oven cavity through the unit.

For normal smoke loads and a metered primary air flow into the first stage, complete combustion will take place therein. In the event of heavy smoke loads and the same metered primary air fiow there will not be enough oxygen available for complete combustion in the first stage. Partially reduced smoke will be discharged from the first stage and the temperature of the first stage ceramic will drop. Fresh air is sucked into the secondary air inlet opening 33 and is available to the second stage for complete combustion of the gases therein.

A better understanding of the operation of the present inventive design can be had by studying the time-temperature graph of FIGURE 3 for one specific embodiment of the invention, where the time is plotted in minutes along the abscissa and the temperature in degrees Fahrenheit is plotted along the ordinate. This graph includes four curves labeled A, B, C and D. Curve A is a curve showing the change of temperature of the oven air or the temperature of the gases approaching the oven vent opening 11 and the first stage of the oxidation unit. Curve B shows the change of temperature of the ceramic block 17 of the first stage Where it lags the oven air temperature at first and then rises above the oven air temperature in the vicinity of 390 F. at point a when the first stage starts combustion, in one specific example of the present invention, to a maximum of about 1100 F. at point b where the first stage becomes overloaded due to a very heavy load of grease of about 8 teaspoons of shortening and the lack of a sufficient supply of oxygen. The first stage stops combustion and its temperature drops rapidly as some of the rich mixture of smoke passes through the first stage. Curve C shows the change of temperature of the oven exhaust gases, leaving the oxidation unit, while curve D shows the temperature of the ceramic block 18 of the second stage and its almost inverse relationship with respect to curve B of the first stage during the overloaded condition. When the temperature of the first stage drops suddenly at point b from the first crest of 1100" F. due to incomplete combustion, the temperature of the second stage increases rapidly at point 0 because combustible products emanating from the first stage start to oxidize here. At point d the first stage starts combustion again due to the correct mixture of smoke and oxygen and at point e the second stage stops combustion. Finally at point i the combustion has been completed and the smoke has vanished. Then the temperatures of the two stages tend to equalize at about 900 F. for the first stage which is the source as the oven air temperature, and 500 F. for the second stage because of the flow of fresh air entering between the stages.

Having described above a specific embodiment of the present invention, it will be understood by those skilled in this art that while the starting temperature of the exothermic reaction has been located somewhere between 300 F. and 400 F., the particular temperature in a given situation will depend upon the following factors: the kind of smoke and undesirable gases being handled by the oxidation unit, the type of catalyst being used, and both the kind and area of contact of the supporting substrate such as the ceramic material.

Modifications of this invention will occur to those skilled in this art, therefore, it is to be understood that this invention is not limited to the particular embodiments disclosed but that it is intended to cover all modifications which are within the true spirit and scope of this invention as claimed.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A two-stage self-sustaining catalytic oxidation unit for receiving the exhaust gases from a self-cleaning pyrolytic cooking oven, said unit comprising a first stage with a tubular housing supporting a catalytic coated perforated ceramic block of cellular construction arranged transversely of the housing for receiving combustible products therethrough, and a second stage also having a tubular housing that is supported in spaced tandem relation with the first stage, a second catalytic coated perforated ceramic block of cellular construction positioned within the second housing, and secondary air inlet means between the two stages so that under heavy smoke load conditions partial combustion takes place in the first stage and complete combustion is insured in the second stage.

2. A two-stage self-sustaining catalytic oxidation unit for receiving the exhaust gases from a self-cleaning pyrolytic cooking oven, said unit comprising a first stage with a hollow housing having an inlet opening and an exhaust opening, a perforated ceramic block of cellular construction arranged transversely of the housing for receiving hot gases through the cells thereof, a catlaytic coating material covering the surfaces of the ceramic block that are exposed to the hot gases, the ceramic block becoming heated by the hot gases whereby at a minimum block temperature between about 300 F. and 400 F. the gases undergo an exothermic reaction until most of the smoke and other undesirable components of the hot gases are oxidized, and a second stage oxidation unit of similar construction to the first stage and arranged in tandem therewith, means for introducing secondary air into the inlet end of the second stage, so that for very heavy loads of smoke accumulation when the first stage may become overloaded and allow smoke to pass therefrom, the exhaust gases mix with the secondary air and enter the second stage and are again oxidized until all smoke and other-undesirable properties in the gases are eliminated. l

3. A two-stage self-sustaining catalytic oxidation unit as recited in claim 2 wherein the first stage has an inlet opening and an exhaust opening, the exhaust opening having a gradually tapered restricted throat portion, the second stage also having an inlet opening and an exhaust opening, said second stage inlet opening having a gradu ally tapered enlarged throat portion telescopically arranged over the exhaust opening of the first stage but spaced therefrom to establish an annular air inlet opening for mixing fresh air with the exhaust gases expelled from the first stage.

4. A two-stage self-sustaining catalytic oxidation unit for receiving the exhaust gases from a self-cleaning pyrolytic cooking oven, said unit comprising a tubular housing supporting a pair of longitudinally spaced catalytic coated perforated ceramic blocks of cellular construction arranged transversely within the housing for receiving hot gases therethrough, a hot gas inlet opening in one end of the housing upstream of the first ceramic block, an air inlet opening in the housing between the two ceramic blocks, and a hot gas exhaust opening at the other end of the said housing. j

5. A two-stage self-sustaining catalytic oxidation unit for receiving the exhaust gases from a self-cleaning pyrolytic cooking oven, said unit comprising a first tubular housing containing a catalytic coated perforated ceramic block of cellular construction, a second tubular housing supported from the first housing in tandem re lation and containing a similar catalytic coated ceramic block, the first housing having an inlet opening and an exhaust opening, the second housing also having an inlet opening and an exhaust opening where the second inlet opening is telescopically arranged over the first exhaust opening, there being a fresh air inlet means at the second inlet opening, wherein partial combination of all undesirable properties in the gases may take place in the first housing and further combustion occurs in the second housing.

6. A two-stage self-sustaining catalytic oxidation unit for handling heavy loads of hot exhaust gases from a self-cleaning pyrolytic cooking oven, said unit comprising a first stage with a first housing containing an exhaust gas inlet, a catalytic coated perforated ceramic block of cellular construction arranged behind the said inlet for receiving the exhaust gases therethrough, an exhaust gas outlet arranged downstream of the said ceramic block, the unit also having a second stage with a second housing containing an exhaust gas inlet positioned to receive the discharge from the exhaust gas outlet of the first stage, a secondary air supply available to the exhaust gas inlet of the second stage to provide make-up air for the second stage, a second ceramic block similar to the first said block arranged downstream of the exhaust gas inlet of the second housing, said second housing having an exhaust gas outlet downstream of the said second ceramic block whereby the hot gases leaving the second stage are at a reduced temperature from the temperature of the hot exhaust gases entering the first stage.

References Cited UNITED STATES PATENTS 2,664,340 12/1953 Houdry. 2,879,862 3/1959 Burden. 3,211,534 10/1965 Ridg way.

MORRIS O. WE'LK, Primary Examiner.

M. D. BURNS, Assistant Examiner.

US. Cl. X.R. 

